The hookload, as measured from the deadline anchor tension, contains contributions not only from the weight of the drillstring, but also from friction in the sheaves. For example, GB 2 275 778 teaches a method for use during the drilling of a well with a drillstring suspended from a hook for determining the load on the hook (hookload), the hook being supported on a rig by a cable and a travelling block, the method comprising: measuring the tension in the cable, typically the deadline anchor tension, measuring a displacement of the travelling block, and the velocity of the travelling block during displacement so as to estimate the friction and using the estimate of friction and the measured cable tension to determine the load on the hook. The arrangement uses a deadline tension sensor, a crown block encoder to measure displacement of the travelling block, and also (since the rig is a floating rig) a slip joint encoder to measure riser extension and a sensor of displacement of the motion compensator. In GB 2 275 778 methods are taught for correcting for some of these frictional effects. The methods differed between rigs with a passive motion compensator (floating rigs in general) and those without.